In general, semiconductor devices require high quality in electrical and mechanical characteristics of dielectric strength, durability, and thermal radiation, however, it is rather difficult to provide an economical manufacturing process of high quality devices in a practical and available form. In a structural view of electronic devices, packaging of a semiconductor element and a support pad therefore is now very important, since the electric and mechanical features in fact depend on the packaged structure of the devices.
Semiconductor devices are widely used in various types of electronic products, consumer products, automobiles, integrated circuit cards, and the like. One feature of semiconductor devices which is important in many of these applications is the small size of a semiconductor device, which includes both the semiconductor die and the package in which it is housed. Keeping device dimensions as small as possible is not only important to single chip devices, but to multiple chip devices as well. However, there is a competing desire for additional I/O's which tends to increase the overall size of the semiconductor device or results in the device having a very fine lead pitch which makes it difficult for the end user to handle.
In addition to establishing a small device size, manufacturers are also driven to maintaining a low cost of manufacturing devices. A significant material cost in manufacturing a semiconductor device other than a semiconductor die is the lead frame. For many devices a customized lead frame must be designed and manufactured for each semiconductor die which is both costly and time consuming.
Resin encapsulated semiconductor devices are usually packaged by either one of two methods. In one method, a semiconductor die, or a plurality of dies, is placed in a package which is then individually mounted on a circuit substrate. In an alternate method, a semiconductor die, or a plurality of dies is mounted directly on the circuit substrate and then is provided with a protective encapsulation structure. The first mentioned method has the advantages that the die is sealed in and protected by the package. The packaged device is easy to test, handle, and install and the encapsulating package provides the desired degree of protection against the environment. In contrast, the second described method in which the die is connected directly to the substrate minimizes the area required by the die and thus allows a very high substrate packing density. In this method, however, an unpackaged die is less easily handled, tested, and burned in, and is more subject to undesirable effects of the environment.
Semiconductor devices may be packaged by several methods commonly used to complete an exposed die which has been directly connected to the substrate. These methods include "overmolding" such as disclosed in U.S. Pat. No. 5,239,198 issued Aug. 24, 1993; "shell and gel" such as disclosed in U.S. Pat. No. 5,095,359 issued Mar. 19, 1992; and the use of premolded plastic packages.
Current overmolded semiconductor packages use epoxy based material formed as a package or device body through transfer or injection molding. Epoxy based molding material has to be kept in the freezer (at approximately -40.degree. C.) and preheated for use. The shelf life is short, a few hours. Unused material has to be discarded. Both early cured molding material as well as material along the runners of the mold must be discarded which accounts for approximately 30-40% of the total molding material. After molding, the package has to be cured for an additional 2 to 3 hours. Transfer or injection molding also requires special equipment at additional cost. To leverage the investment on the equipment and the cost for the special mold, high volume is required (for example greater than 1,000,000 units a year) to justify a new package. Finally transfer or injection molding require the use of fairly substantial heat and pressure in the molding process, which may not always be desirable.
The semiconductor devices produced by other two common approaches also have their disadvantages. For example, the shell and gel approach results in a higher profile (larger dimensions) than that which may be obtained by molding to fit, and may be difficult to fit into the smaller thinner modules which are being demanded by the current market. Premolded plastic packages also have difficulties with size and, perhaps more importantly, again require high volumes to justify the cost of creating new molds and new packages specifically designed for the semiconductor device to be used.
The current systems do not provide an acceptable alternative combining low start-up and change-over costs with the ability to do low volume to high volume packaging while maintaining an acceptable profile for the overall package.